Recently, concentration measurements of particulate substances such as PM 2.5 are frequently performed. A mass of particles in gas per a unit volume is used as a concentration unit of particles in gas. The particles concentration is a mass concentration. There is a method of collecting particles of gas into a filter and measuring a mass of the particles as a standard method of measuring the mass concentration of the PM 2.5 (for example, see Japanese Patent Application Publication No. 11-502303). There is a beta-ray absorption method as a method of measuring a mass concentration that can be automatically performed. The concentration obtained by the filtering method or the beta-ray absorption method is a mass concentration. Presently, the PM 2.5 concentration is generally expressed as the mass concentration. Moreover, there is a method of light-scattering detection method of measuring the number of particles in gas with use of a scattered light obtained by radiating a light to the particles in the gas, as a simple method.